Centrifugal pump units of this type, when they are equipped with wet-running motors, are predominantly used as circulating pumps in heating systems. As the generated power of the vast majority of these units is typically below 100 watt, the motors have a very small starting torque. Both in closed systems, such as heating installations for example, and also in open systems, contamination of the conveyed fluid can cause problems. The contamination can be in the form of particles of dirt, deposited particles of lime or other substances in suspension. If such particles reach the centrifugal pump unit, then as a rule this is still not a problem, as long as the unit is kept in operation. However, if the pump unit is brought to a standstill, then the particles situated inside the pump unit can easily lead to the clogging, i.e. jamming, of the rotor, so that when the unit is switched on again, the rotor is not able to rotate. The cause of this jamming of the rotor may be particulate solids borne by the conveyed flow or deposits (e.g. lime) on the rotor shaft. These particles or deposits may be drawn into the bearing gap through axial movement of the rotor brought about on switching off. Because bearing tolerances are nowadays kept as small as possible, for reasons of noise development, even minute particles can increase the friction moment inside the bearing to such an extent that the starting torque of the motor is not sufficient to set it in motion.
A typical pump of the type described above is known for example from DE 32 10 761 C1. In this publication, the above-mentioned problem is likewise described. To solve this problem, the pump unit described therein--like almost all pump units of this type--has an opening in the base of the split tube casing and also of the pump housing, which is able to be closed by means of a plug. Through this opening, the end of the rotor shaft of the motor is freely accessible after the plug is removed. Through this opening, a screw driver can be inserted into a groove on the end face of the rotor shaft, and turned to unjam the pump.
This manual unjamming generally leads to the desired result, but has the disadvantages that it is time-consuming, and should be carried out by specialist personnel. This leads to delay and increased costs.
However, even the specialist is possibly in danger during the activities described above for unjamming the pump. Through the removal of the plug, the customarily pressurized water-carrying system, is opened, so that it is possible that very hot water may emerge through the bearing gap under pressure into the environment.
A further disadvantage is the fact that in typical modern heating systems, three such pump units are integrated in one installation, which are switched on or off automatically through a regulating or controlling the installation. Here, the risk of blocking and breakdown of the entire installation is particularly great owing to the frequency of switching pumps on and off.
In addition, it is structurally disadvantageous to provide a closable opening, in the base of the split tube pump casing and also in alignment therewith in the motor housing. Additional processing stages and components (plug, seal etc.) are necessary. Also the required accessibility of the opening restricts the fitting position, which is a disadvantage, particularly in compact heating installations.